The present invention relates to a process for the recovery of antimony pentachloride from exhausted catalyst solutions. More particularly, the process of the present invention yields antimony pentachloride from used catalyst solutions such as those employed in the fluorination of chloro-hydrocarbons.
It is known that many highly technical processes for the production of chloro-fluoro-hydrocarbons, especially chloro-fluoro-methane and chloro-fluoro-ethane, include reacting the chlorine containing starting compounds with hydrogen fluoride in the presence of antimony pentachloride catalysts to bring about the substitution of a portion of the chlorine on the hydrocarbon compound with fluorine. The compounds produced according to these technical processes have achieved great importance as solvents and coolants as well as being introduced as propellants in the aerosol industry. However, the methods for producing these important products require large amounts of expensive antimony pentachloride catalysts because the catalysts are used up in the process of fluorination of chloro-hydrocarbons.
The lifetime of the utilized catalyst solution is limited according to various circumstances. It is considerably dependent on external influences as for example on the purity of the raw products introduced or the kind of catalyst reaction. Because of the presence of impurities in the charged reaction components, high boiling organic compounds are often formed. These high boiling organic compounds can accumulate in the reaction system in large concentrations because their low vapor pressure, under the reaction conditions, permit only very slight quantities to be removed by distillation and thereby dilute the catalyst solution. Further, water can be introduced into the reaction system with the reaction components that are not absolutely water free. Subsequently, this can bring about a concentration of highly corrosive and only slightly decomposible antimony chloride-water complexes. Moreover, through corrosion of the apparatus and equipment parts, iron antimony complexes developed in the system which are not decomposible. The introduction of foreign material, whether of organic or inorganic nature, brings about a dilution and partial inactivation of the antimony pentachloride in the catalyst solution and correspondingly results in a diminishing rate of reaction and consequent loss of efficiency. Accordingly, the exhausted catalyst solution must be exchanged for chemically pure antimony pentachloride. The high price of reaquisition for antimony pentachloride and the extremely expensive disposal of used catalyst solutions make the recovery of antimony pentachloride extremely desirable.
Attempts at recovery of antimony pentachloride from used catalyst solutions through direct distillation have not been satisfactory, even with costly apparatus, because the high boiling organic compounds in the catalyst solution have boiling points which partially overlap with the narrow boiling range of antimony pentachoride. The consequent interference with the distillation of antimony pentachloride results in an ineffiecient, time-consuming operation.
According to other earlier known methods for the recovery of antimony pentachloride from used catalyst solutions, the antimony chloride salts present in the catalyst solution are converted into SbCl.sub.3 and this is separated from the organic solution.
Such a procedure is described in German published application 2,056,648 wherein the used catalyst solution is heated under the addition. of an equal volume of trichloro-ethylene in an autoclave at 100.degree.-170.degree. C followed by the separation of the crystaline antimony trichloride from the interferring contamination after cooling. A highly technical conducting of this method is not economical because this purification process must be carried out in an expensive pressure apparatus of nickle and because it utilizes great amounts of solvent. Further, complete recovery of the starting antimony is not possible in this procedure. The recovered SbCl.sub.3 must be finally converted by way of chlorination into SbCl.sub.5.
German published application 2,110,797, and its corresponding U.S. Pat. No. 3,784,671, propose the thermal dissociation of the SbCl.sub.5 in the catalyst solution into SbCl.sub.3 and Cl.sub.2, extracting the organic contamination with haloginated hydrocarbons, subsequently oxidizing the SbC1.sub.3 contained in the residue with chlorine to SbCl.sub.5 and distilling off the recovered SbCl.sub.5 in a vacuum from the inorganic impurities. This method also has the disadvantage of being uneconomical because, as the working example shows, comparatively high temperatures are necessaary to achieve the thermal decomposition of SbCl.sub.5 to SbCl.sub.3 and Cl.sub.2. The procedure for carrying out this thermal reaction as well as the following chlorination of the SbCl.sub.3 requires a pressure type apparatus of nickel alloy. The utilization of steel is not recommended for safety reasons of cause of the thermal stress in the presence of chlorine. At the high temperatures of the thermal decomposition, there exists a possibility that a portion of the SbCl.sub.3 undergoing sublimation, as well as part of the high melting organic compounds, would be pulled out with the large amounts of chlorine escaping from the apparatus, resulting in the possibility of encountering dangerously high pressures because of a plugging of the chlorine gas escape passageway. Further, the separation of SbCl.sub.3 from the organic impurities requires great amounts of carbon tetrachloride and other solvents as well as a large expenditure of time and energy. The corresponding separation in this process by way of recrystallization of the SbCl.sub.3 has the disadvantage that it must be repeated several times to enable recovery of sufficient quantities of antimony trichloride. Because great quantities of dissolved SbCl.sub.3 are separated also with the solvent, the SbCl.sub.3 must be further recovered by distillation of the solvent from the organic extracts. A distillation recovery of the dissolved SbCl.sub.3 from the combined solvents can only be successful with a very costly distillation apparatus because, for example, hexachloro-ethane boils only 35.degree. C lower than antimony pentachloride, and tends to sublime at this temperature. Considering the required quantities of solvents utilized and the quantity of antimony trichloride recovered from this extraction material, this method is recognized to be uneconomical.
According to the method described in the German published application 2,140,188 and its corresponding U.S. Pat. No. 3,806,589, the used antimony halogen salt catalysts are separated through extraction with water from the halogenated hydrocarbons. For purification, the antimony chlorides are reduced, for example, with sulfur dioxide in the presence of alkali iodide to SbCl.sub.3. For the removal of the heavy metal ions and the other foreign ions present, the SbCl.sub.3 is converted with a concentrated ammoniacal solution into antimony trioxide. After reconversion of the filtered-off antimony oxides into SbCl.sub.3, very pure SbCl.sub.3 can be obtained through fractional distillation. According to this procedure, the reduction of SbCl.sub.5 requires only a relatively modest expenditure with a low cost reduction material. The method, however, has the same disadvantage as in the previously discussed processes that generally a reduction of the SbCl.sub.5 SbCl.sub.3 must be performed in the process.
In the exhausted catalyst solutions the antimony exists for the most part in a form as active SbCl.sub.5. The inactivity of the catalyst solutions rests, to a considerable measure, upon the dilution of the antimony pentachloride by high boiling organic compounds. The removal of the high boiling organic compounds from the SbCl.sub.5 is extremely difficult to perform on an efficient economical basis. According to earlier known methods great quantities of Sb.sup.5.sup.+ are converted into Sb.sup.3.sup.+, which thereafter must be reoxidized.
There exists, therefore, a definite need to develop a process for the recovery of antimony pentachloride from used catalyst solutions in a simple economical, efficient method suitable to yield antimony pentachloride ready for use again as a catalyst.